1. Field of the Invention
This invention relates to processes for heating materials (preferably dielectric ceramic materials) by means of microwave energy. More particularly, the invention relates to a process for preparing a heat-treated body from a material that does not couple well with microwaves but which nevertheless uses microwaves for the heating step.
2. Description of the Prior Art
The use of microwave energy rather than conventional thermal energy in industrial processes is becoming more widespread because of the rapid and economical heating that can thereby be achieved. However, many materials are either transparent to microwave energy or have low coupling efficiencies so that microwave energy cannot be used to heat these materials directly. In these cases, microwave susceptors are sometimes used to make microwave heating possible. Susceptors are materials that couple well with microwaves and thus generate heat when irradiated. If the susceptors are positioned close to the non-susceptible material, the latter is heated by conduction and/or radiation.
While this procedure is acceptable in many cases, it is not suitable when products of very high purity are required, e.g. sintered ceramic bodies for use in the electronics industry. If the susceptor is mixed directly within the body of material, it remains in the body after the heat treatment has been carried out. Alternatively, if the susceptor is used in the form of a bed surrounding the body, the susceptor contaminates the surface of the body and may in some cases penetrate into the interior. This can be seen from the prior art references discussed below.
Nishitani U.S. Pat. No. 4,147,911 issued on Apr. 3, 1979 discloses a method in which a dielectric material is mixed with 0.05 to 10% by weight of metal powder or other susceptor so that the resulting body may be heated and sintered by microwaves. The problem of contamination of the resulting product by the susceptor is recognized in this document itself (see column 3, line 64 to column 4, line 4), but the only suggested solution is "to carefully investigate the amount, particle size and quality of the substance to be added."
Sutton et al U.S. Pat. No. 4,219,361 issued on Aug. 26, 1980 also relates to the use of a susceptor with a non-susceptible material, but in this case the susceptor may be formed in situ by reactions which take place prior to the microwave heating step. Neverthless, contamination of the product remains.
Levinson U.S. Pat. No. 3,585,258 issued on June 15, 1971 describes the use of a susceptor both in and around the ceramic body to be treated, but again there will be contamination of the final product with the susceptor.
Accordingly, there is a need for a process for enabling non-susceptors to be heated by microwaves without contaminating the final product with an undesired material, and it is an object of the present invention to provide such a process.